If really embedded, you may not be able to polish out. But for more sufferficial surface build-up, I’ve had reasonably good results with common 409 cleaning spray. Spray on, let sit for 5 minutes. Repeat if you seem to be making head-way. This issue made us totally give up on ZYP and boron in general. Life has been much better (for me & spouse) with better kiln wash and kiln wash technique. The haze left by boron is unacceptable on most pieces.